1. Field of the Invention
The invention relates generally to oil and grease resistant packaging. In more specific aspects, the invention relates to methods for applying oil and grease resistant overprint varnishes (OPV) to multi-wall packaging substrates and to the resulting packaging end product.
2. Description of the Prior Art
Consumer packaging is very important in today's market place. Not only do the products within the packaging have to be fresh, but the package outside must be aesthetically appealing. Traditionally, to get the best protection for the goods within packaging, the outer packaging quality was sometimes sacrificed.
Most consumer packaging is made from single ply substrates that are added together to form multi-ply or multi-walled flexible packaging. Some paper suppliers manufacture multi-wall natural kraft paper substrates, but most packaging producers make their own multi-wall constructions. Typically, fluorocarbon treated, bleached, clay coated or unbleached natural kraft paper is a secondary oil and grease barrier for such multi-wall paper packaging. For aesthetic reasons, this paper substrate must be oil and grease related stain resistant and “stain proof” from both inside and outside of the package. Stain resistant materials prevent oil and grease from bleeding through a substrate, which makes a stain resistant material a physical barrier for oil and grease. Stain proof materials do not show any oil and grease staining. The oil and grease may pass through the fiber matrix within the substrate, but stain proof materials will not show a stain. This stain proof requirement exists in manufacturing, packing, shipping and handling environments. Many times the stain proof substrate is the outer print ply of the packaging.
In addition to being stain proof, packaging typically must have some graphics printed on the exterior ply in order to identify the contents of the package and for advertising purposes. The paper substrate has to be printed with the information. To enable the graphics to remain on the substrate without smearing or fading, an overprint varnish is typically applied to the outer ply of the bag.
For years, most oil and grease resistant coatings contained chemicals known as fluorocarbons. These fluorocarbon based coatings are paper machine applied saturants. Examples of this type of coating are described in U.S. Pat. No. 5,674,961, issued to Fitzgerald and U.S. Pat. No. 4,097,297 issued to Keene. Synthetic fluorocarbons have recently come under governmental scrutiny due to their deleterious environmental effects. Fluorocarbons may be considered toxic and may be considered non-biodegradable, which arguably results in bioaccumulation within humans and other mammals. An alternative non-fluorocarbon, multi-wall substrate outer print ply is needed for the oil and grease resistant (OGR) marketplace.
Prior attempts have been made to decrease the environmental effects of fluorocarbons. More environmentally friendly alternative synthetic fluorocarbon paper treatments have been developed. Problems still exist with these alternatives, however. Many of the alternatives still contain a fluorocarbon content. Since the fluorocarbon content is so low, these alternatives may be considered less persistent in the environment, but there are still issues with toxicity to humans and other mammals. Additionally, the fluorocarbon alternatives are expensive to produce, and it is uncertain if a reliable supply source exists for the future.
One example of a more environmentally friendly overprint varnish is described in U.S. Pat. No. 4,097,297 to Keene (the '297 patent). The '297 patent describes an overprint varnish that is nitrocellulose based. The overprint varnish, however, still contains fluorocarbons in a small percentage in its composition. This overprint varnish works better when used in a solution containing a solvent. The use of a solvent, however, adds considerable costs to the manufacturing process and the solvents emit harmful volatile organic chemicals (VOC's) during application. Additionally, the method of drying the overprint varnish to substrates is inefficient and time-consuming. The drying process requires either drying with a heater, which increases capital costs, or at room temperature, which increases the time required to dry the overprint varnish.
To combat the environmental and toxicity consequences of using fluorocarbons, non-fluorocarbon alternatives have been developed. Such developments include synthetic latexes, a combination of natural cellulosic gums, starch, and/or proteins, and heavy paper refining. While these alternatives are certainly more environmentally friendly and less toxic than fluorocarbons, there are still problems using these alternatives. The use of synthetic latexes or the combination of natural cellulosic gums, starch, and/or proteins requires very high coat weights when applying them to the paper substrates. The high coat weights result in poor paper machinery operation and increase the amount of maintenance required on these machines. Each of the alternatives are very costly, which makes them economically infeasible. Additionally, none of the three alternatives provide very good oil and grease resistance.
Problems also exist related to processes for applying overprint varnishes to substrates. U.S. Pat. No. 6,132,822 to Overcash, et al. (the '822 patent), for example, discloses a method of applying an overprint varnish that is environmentally friendly and has good oil barrier characteristics. The application process disclosed in the '822 patent, however, requires a high coat weight and that the high gloss coating to be rod applied. These limitations make this process very expensive and inefficient since it is difficult for the paper machines to handle, as previously discussed. Additionally, the process is limited since other application methods, such as flexography, cannot be used.
As another alternative to fluorocarbon components, plastic and polymer alternative packaging constructions have been developed. Problems also still exist with these alternatives, however. The plastic and polymer packaging materials have very high raw material costs associated with them. These alternatives also operate less efficient on the converting and packing machinery. Additional equipment is also necessary when using these alternatives. This increases the capital assets that are required, which many times makes use of these alternatives economically unfeasible.
Most packaging producers purchase paper substrates to make their packaging that are pretreated with oil and grease resistant materials, such as those described above. The prior art oil/grease resistant coatings are applied to the substrates prior to construction of the packaging. Once they receive the oil and grease resistant treated substrates, they then apply their graphics to the substrates, apply an overprint varnish over the graphics, and then form their end packaging products. This increases the raw material costs in manufacturing their products since oil and grease resistant coatings increase the cost of the paper substrates.
With more and more products being packaged for consumer convenience, a need exists for packaging that is environmentally friendly, and provides superior barrier properties to oil and grease. With increased market competition, the packaging preferably needs to be able to be decorated aesthetically and possess enough strength to withstand the elements to which the packaging will be exposed. A further need exists for a method of applying an overprint varnish to paper substrates that is more cost effective, efficient, and easier to run on the existing paper processing machinery. Since an overprint varnish is applied typically to substrates before forming end product packaging, another need exists for simplifying this process by including the oil and grease resistant properties in the overprint varnish and eliminating the need to purchase oil and grease resistant substrates.